Support NeoGAF

[BTMash]

Edit - I've been working on some of my physics homework and have come across questions which I'm unsure on how to solve. Any help would be appreciated

1. A rectangular box is placed on a table on Earth. The top of the box is negatively charged. The bottom of the box is positively charged. An object of mass m and charge q is placed at the centre of the box. If the object remains suspended at the centre of the box, explain how you would determine the value of q.

2. How do we know that the strength of the magnetic field associated with the strip on the back of a credit card is very weak?

 

[slayn]

1. The force from the attraction/repulsion added togethor has to cancel out the gravitational force. I assume the charge of the top/bottom of the box is the same and they are of equal distance so they would both provide the same amount of upward force. I forget the exact equations to explain any further but thats the idea.

 

[nitewulf]

hmm, late night physics

well, since the object remains suspended in the middle, you know the top and bottom portions are exerting a force equal to the gravitational force acting on the object.
so use whatever equation you have for the force exerted by rectangular charged plates on a point charge (i dont recall at the top of my head) Q, and equate that to the gravitational force F=mgh. and solve for Q.

secondly, yeah how do you know, think about it. how do you know a magnet is a magnet?

 

[slayn]

mgh is potential energy, not force =P

 

[nitewulf]

so, just mg right? i am not sure, been a while since i took classical mechanics.

 

[slayn]

yeah mg. I have no clue on the electrical formulas any more. Only one I can call to mind is two point charges which isn't really helpful here. Thats what physics formula sheets are for! =P

 

[Dilbert]

1. A rectangular box is placed on a table on Earth. The top of the box is negatively charged. The bottom of the box is positively charged. An object of mass m and charge q is placed at the centre of the box. If the object remains suspended at the centre of the box, explain how you would determine the value of q.

Assuming the dimensions of the box are large with respect to the size of the charged object and that the box is empty, you have a parallel plate capacitor. If E is the electric field between two charged plates, a particle with charge q experiences a force of qE in that field. We also know that the gravitational force for an object of mass m is F = mg. So, from equilibrium, we have mg = qE, and therefore q = mg/E.

(By the way...this is exactly Millikan's oil drop experiment.)

2. How do we know that the strength of the magnetic field associated with the strip on the back of a credit card is very weak?

Ummm...I don't know what the right answer is, but here's the first thing which comes to mind. If you take two strong bar magnets and lay one on top of the other at an arbitrary angle, torque is generated since the magnetic fields want to align. (You probably did this as a kid.) On the other hand, if you lay two credit cards on top of each other, you don't experience any torque at all. (Which is good -- if the fields were strong enough to interact, then you'd wipe out all of your information when you stack three or four of them together in your wallet!)